1. Field of the Invention
This invention relates to pipette tips capable of dispensing fluids, and more specifically relates to pipette tips used in wet and dry chemical analyzers capable of precisely dispensing minute quantities of fluid.
2. Description of the Prior Art
FIGS. 1, 2 and 3 are various views of a conventional pipette 2 incorporated in the VetTest® veterinary blood analyzer marketed by IDEXX Laboratories, Inc. of Westbrook, Me. The conventional pipette includes a main body 4 having a central bore 6 extending axially therethrough and a dispensing tip 8. The dispensing tip 8, having a distal tip end 9, opposite proximal end 11 and sidewall 13, has a circular opening 10 formed in the bottom surface of the distal end 9 leading to the central bore 6 of the pipette main body 4. Fluid is dispensed from the central bore 6 out the circular opening 10 by means of pneumatic force. The structure and operation of this pipette is more fully described in U.S. Pat. Nos. 5,089,229; 5,250,262; and 5,336,467, each of which issued to Thomas Heidt et al., the disclosures of which are incorporated herein by reference.
The VetTest® system is used to apply body fluid, e.g., urine, serum and/or plasma onto test slides having a chemical or biological reagent on their surface. The conventional pipette 2 automatically distributes amounts of fluid onto a plurality of test slides, each of which may have a different reagent coating. A minor concern with the VetTest® apparatus is that occasionally there is a spot failure (i.e., an improper application of serum/plasma to a slide). This infrequent spot failure may result from inconsistent volumes of fluid or no fluid being deposited on the test slides. Spot failure has been at least partially traced to the design of the pipette tip and the material (i.e., polypropylene) from which the dispensing tip 8 of the pipette is preferably made.
As a specific quantity of fluid is dispensed from the dispensing tip 8 of the conventional pipette 2, the fluid dynamics cause a generally spherical droplet to form at the circular opening 10. As the droplet nears the desired volume, the pipette 2 is lowered toward the slide until the droplet just contacts the chemically coated film portion of the slide, whereupon it is drawn from the dispensing tip 8 of the pipette 2 due to capillary action, surface tension and gravitational force on the droplet. Unfortunately, the ability to control the exact amount of fluid dispensed onto a test slide is difficult, as occasionally small amounts of the droplet are drawn up over the outer surface of the distal tip end 9 of dispensing tip 8, which is due at least in part to the propensity of the polypropylene pipette tip 8 to “wet” or attract fluid on the outer surface of the pipette tip. Wetting the outer surface may cause an imprecise volumetric quantity of fluid or no fluid to be deposited on the test slide.